Method and apparatus for reducing echo effects in picture transmission systems



Search Roon 358-187. GR 293869087 SR Oct. 2, 1945. F. J. BINGLEY ETAL2,386,087

METHOD AND APPARATUS FOR REDUCING ECHO EFFECTS IN PICTURE TRANSMISSIONsYsTEMs Filed larch 6, 1942 6 Sheets-Sheet l DIEK ELWd Z 1 -1-1 r'scwobaseman/vin l:

IELEGRAPHY, ua UUH 1945. F. J. BINGLEY EI'AL 2,336,087

METHOD AND APPARATUS FOR REDUCING ECHO EFFECTS IN PICTURE TRANSMISSIONsYs'rEus Filed March 6, 1942 6 Sheets-Sheet 2 name 121.

178. ItLtbHA'rHY,

Oct. 2, I945. F. J. BINGLEY EI'AL 2,386,087 METHOD AND APPARATUS FORREDUCING ECHO EFFECTS IN PICTURE TRANSMISSION SYSTEMS Filed larch 6,1942

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METHOD APPARATUS FOR REDUCING E EFFECTS PICTURE TRANSMISSION SYST Filedllaroh 6, 1942 6 Sheets-Sheet 5 J Ma a a m ww S Hill.

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Patented Oct. 2, 1945 053151553 iiU'U UNITED STATES PATENT OFFICE METHODAND APPARATUS FOR REDUCING ECHO EFFECTS IN PICTURE TRANSMIS- SIONSYSTEMS Application March 6, 1942, Serial No. 433,660

35 Claims.

This invention relates to picture transmission systems and the like, andto a method and means for substantially reducing or eliminating theeffects of echo signals on the received or reconstituted picture. Moreparticularly, the invention relates to television systems, and to amethod and means for reducing or substantially eliminating certainundesirable effects resulting from the arrival, during the picture lineperiods, of echoes of the horizontal or line synchronizing signals.

One of the problems encountered in picture or television transmission,and one that obtains with any type of modulation, is that which resultsfrom the reception of long-delayed echoes, corresponding to signal pathdifferences of the order of a mile or several miles. Since the speed ofpropagation of the radio wave is approximately 1000 feet permicrosecond, a path difference of say three or four miles, as betweenthe direct and the reflected signal path, may produce echoes delayed byfifteen or twenty microseconds. In magnitude these echoes may berelatively faint due to the greater path length, to the fact that theecho-signal normally sufiers considerable attenuation in reflection, andto the fact that the indirect path is usually closer to the earth thanis the direct path. On the other hand, since the blankin andparticularly the synchronizing sig nals are of greater amplitude thanthe line or picture portion of the signal, reflected blanking andsynchronizing signals of considerable strength sometimes appear in thepicture as a result of these long-delayed echoes.

In general, an echo may appear in a television picture by combining orbeating with the picture carrier. Thus, the echo may add to or subtractfrom the picture carrier depending upon the particular phase relationbetween the two signals at a given instant. This phase relation depends(except for changes effected at the transmitter itself) upon thedifference in path length between the direct and reflected signals; andthe only way in which this difference can be appreciably varied,assuming that the transmitting and receiving antennas are fixed, is by avariation in the placement of the reflector causing the echo. But sincethe reflector is usually a high building, bridge, gas tank, hill, orsimilar object, it too can be regarded as a substantially flxedstructure, and, hence, in any given installation the echo signal, as itappears upon the viewing screen at the receiver, is subject only to suchphase relations between the direct and reflected signals as may resultfrom periodic phase changes effected within the transmitter itself.

We have found that the deleterious effects of these echo signals may besubstantially reduced or eliminated by periodically changing thepolarity of the echoes seen at the receiver, so that they are oppositein successive frames. This can be accomplished most readily byperiodically changing the phase of the echo carrier with respect to thepicture carrier, these changes being so timed that the successive echoesbalance each other out so far as their impression upon the observer isconcerned. The phase changes referred to are effected at thetransmitting station and require no additional equipment at the point ofreception.

It is a principal object of this invention to provide a method and meansfor substantially eliminating certain echo effects which may beencountered in picture transmission systems.

Another object of the invention is to provide a method and means fortransmitting a television signal, or the like, which when received willproduce a picture which is substantially free of echoes of thesynchronizing signals.

Still another object of the invention is to provide means foreliminating certain of the spurious images produced when televisionsignals are received by way of two or more transmission paths.

A further object of the invention is to provide a method and means bywhich the relative phase of echo signals may be periodically advanced,retarded, or otherwise changed, the individual changes themselves beingof such a magnitude that, if continued in a given sense, a smallintegral number of the said changes would produce an effective echophase rotation of substantially 360 degrees.

These and other objects and features of the invention will be apparentfrom the following description and the accompanying drawings, in which:

Fig. 1 shows a typical disposition of transmitter, receiver, andreflecting structure, which may give rise to objectionable echo signals;

Fig. 2 is an explanatory diagram illustrating certain characteristics oftypical echo patterns;

Figs. 3, 4, 5 and 6 are illustrative of some of the methods employed toeffect cancellation of various echo patterns:

Fig. 7 is a diagrammatic representation of television signals which maybe employed to attain the desired objects of this invention;

Fig. 8 is a block diagram of a transmitting system, constructed inaccordance with the invention, for generating a television signal which,

when received, will be substantially free of deleterious echo effects;

Fig. 9 is a schematic diagram illustrating certain of the details of thesystem of Fig. 8;

Fig. 10 is an explanatory diagram illustrating the operation of thesystem of Fig. 8; and

Fig. 11 is a schematic diagram illustrating a three-phase phase-changingcircuit arrangement.

The present invention may best be understood by considering first thecauses of echo signals, and their appearance when viewed on the screenof the picture tube. A typical condition which may give rise toobjectionable echoes is illustrated in Fig. 1. In this figure, atelevision receiver R is represented as being 12 miles distant from atransmitting station T. At distances of 6 and 10 miles from thetransmitter and receiver, respectively, is a wave reflecting structuresuch as a tall building, a water tower, a steel bridge, or the like.Taking the speed of a radio wave as about oneflfth mile per microsecond,it will be apparent that the time required for the signal to traversethe direct 12-mile path between the transmitter and receiver will beabout 60 microseconds, wh-le the time required for the reflected signalto traverse the indirect 16-mile path will be about 80 microseconds.Consequently, the reflected wave, 1. e., the echo, will arrive at thereceiver about 20 microseconds behind the direct wave. This may beregarded as illustrative of a simple echo, as differentiated frommultiple echoes which arise when a plurality of reflecting structuresprovide a plurality of indirect signal paths of different length.

Fig. 2 is an explanatory diagram in which the time and amplitudecharacteristics of a typical television signal S are related to thescreen P of a conventional television picture tube. The televisionsignal, in accordance with the present practice, may comprisesynchronizing pulses S of approximately 5.1 microseconds duration,blanking pulses Sb of approximately 10.2 microseconds, and a line orvideo period Sv of approximately 53.3 microseconds. These aresubstantially the speciflcations employed in a conventional 525-lineinterlaced television system, with 30 frames 60 fields) per second.Since the line period corresponds to the time required for the electronbeam in the picture tube to trace one line across the picture tubescreen, the width of the screen P has, for convenience, been made equalin length to the line portion of the television signal S. As isconventional, the amplitude of the blanking pulses Sb may be regarded ascorresponding to a black signal level, zero carrier as corresponding toa very bright signal level, while the synchronizing pulses may beregarded as corresponding to a blacker-than-black or infra-black" signallevel. The video signal existing during the 53.3 second line period has,for the purpose of this description, been established at a level midwaybetween zero carrier and the black level, and will correspondapproximately, therefore, to a gray level.

When a signal of these characte istics is received without an attendantecho, there will be reproduced, upon the screen of the picture tube. a

line similar to that shown (with exaggerated portion of the signal can,by reason oi. their lesser amplitude, be ignored, and, consequently, formost practical purposes only those echoes produced by the higheramplitude blanking and synchronizing pulses need be considered. Of thesepulses, the latter are, of course, the more important by reason of theirgreater amplitude.

Line B in Fi 2 is illustrative of the normal appearance of a pictureline when receiving a strong echo signal. The echo illustrated resemblesthose which are produced by the delayed reception of the blanking andsynchronizing signal, and is displaced from the left-hand edge of thescreen P by a distance which is proportional to the time intervalbetween the arrival of the signal traveling the direct path and thearrival of the signal traveling the indirect or reflected path. If thedifference in length between these paths is small, the echo will bereproduced at or near the left-hand edge of the screen, whereas greaterpath diflerences cause the echo to appear further to the right. Ofcourse, where the path difference is very great, the attenuationsufl'ered by the reflected signal is usually so considerable that theecho is too weak to be noticeable.

As is indicated in the drawings, the echo shown in line B is produced bythe arrival of blanking and synchronizing pulses during the line orvideo period Sv. Specifically, this echo is produced by the arrival ofthe pulses Se, Sb, by way of an indirect path, approximately 20microseconds after the reception of these pulses by way of the directpath. The echo signal produces on the screen P an echo pattern whosewidth is equal to the width of the line, and whose length is equal tothe distance traveled by the scanning beam in 10.2 microseconds, theduration of the combined blanking and synchronizing signal. The 20-sec-0nd delay chosen for this illustration, it will be recalled, isapproximately the delay produced as a result of a reflected signaltraveling an additional 4 miles as illustrated in Fig. 1.

Whether the echo, as it appears upon the picture tube screen, will be adark echo or a light echo, depends upon the phase relation between thevideo carrier and the echo carrier at the point of detection. If thesecarriers arrive more or less in phase, the resultant R. F. signalsupplied to the detector will be greater than the amplitude of the videocarrier alone, and, consequently, the combined signal will tend towardthe black level and a "dark echo will be produced as shown in line B.The darkest part of the echo will be that central portion whichcorresponds to the synchronizing pulse SS, while the outer portionscorresponding to the blanking signal 4 will be somewhat less dark, butdarker than that part of the line which is not distorted by echo. On theother hand, if the video and echo carriers arrive in generally oppositephase, the reverse will be true, and a light echo such as that shown inline C will result.

Still a different type of echo results when television transmission iscarried out in accordance with the system described by A. V. Loughren(Electronics, February 1940, pp. 27-30), or with the system described inthe copending application of Frank J. Bingley, Serial No. 401,533, filedJuly 8, 1941. In the alternate carrier system disclosed by Blngley, thesynchronizing pulse is transmitted not only as a variation in carrieramplitude (as in Fig. 2), but also as a variation or shift in carrierfrequency. In this system, the synchronizing signal carrier may be ofthe order of 1 megacycle or more higher than the video W8. TELEGRAPHY,

changes, while all even-numbered frames might be so transmitted that thecarrier phase obtaining during the synchronizing (or blanking andsynchronizing) intervals is reversed with respect quency differencebetween these signals, e. g., 1 6 to the carrier phase obtaining duringthe video megacycle. Since this frequency lie within the intervals.video frequency band, the synchronizing signal This may best beunderstood in connection with portion of the echo signal will bereproduced on the composite signal representation of Fig. '7 (a) thepicture tube screen as an alternating" echo, which is intended toillustrate, in reduced scale, i. e., one which alternates from dark tolight to 10 a television signal of the type shown in the lower dark,etc., as shown in line D of Fig. 2. The portion of Fig. 2. The signalsto the left of the number of these light and dark bars depends upondashed line x-a: may be considered as those prothe number of beat cyclescontained within the ducing the scan shown as frame No. l in Fig. 3,duration of the synchronizing pulse. The change While the signals to theright of the dashed line in shading from a dark bar to a light bar. andare those producing the scan denoted frame No. vice versa, is gradual,of course, and not abrupt, 2 in Fig. 3. In each frame the echo isproduced, as shown in the illustration. as hereinbefore described, bythe delayed arrival In general, where strong echo signals are reofechoes of the synchronizing and blankin sigceived, the echo imagesreproduced upon the nals. In Fig. '7 (a) the absence ofcross-hatchscreen of the picture tube are apt to be more ing in thesignals to the left of the dashed line objectionable in an alternatecarrier system than 3-4: i i t nd t i di te that n phase shift in thesin le ca rier syst heretofore mp y is produced in the carrier asbetween the video T is s bec u e n t former sy the y and thesynchronizing and blanking intervals, nizin si s tra a a frequency forwhereas in the signals shown to the right of which the receiverssensitivity is approximately air-x, the cross-hatched blanking andsynchrondouble that in the l er sy nd, conse izing signals are intendedto indicate that the q y. the reproduced echo s t much m phase of thecarrier Wave, during the blanking preneuneed- Thus, While the Presentinvention and synchronizing intervals, is reversed with reis a p d f usein an t p of television system, spect to the phase of the carrier duringthe video its use is especially desirable in alternate carrierintervals, Consequently, if the delayed syny s. 0 e ally in systemswhere the chronizing signal echoes arrive in such phase, sy ni in si nalis transmitted n u a during frame No. 1, as to produce a dark echo, itmanner as to pro unusually Strehg sy follows that during frame No. 2(with relatively i n p s in e c e eirellitsreversed phase relationships)light echoes will be Attention is now directed to Figs- 5 and produced.The characteristic of 'the echo which illustrate certain of the methodswhich (whether dark or light) during t video t m y be p d. n a da w tpresent vals of frame No. 1 is indicated arbitrarily in invention, toeffect substantial cancellation of 7 (a) by the circled plus signs;whereas, eehO Patterns N0 attempt s been made in during frame No. 2,circled minus signs are emthese figures to maintain the identical scaleem- 40 ployed t indicate t t t echo would have a pleyed in Moreover forsimplicity only different characteristic as a result of the reversal theecho caused by the delayed arrival of h in phase relations. Anelectrical system adapted synchronizing signal is illustrated, thelesser to produce the desired phase changes 111 be echo produced by theblanking signal being scribed in detail hereinaiten omitted. Of thenumerous echo cancellation From the standpoint of the observer, t opti.schemes that we have developed, with the present cal fiect produced bythe rapid alternation f inventionas a basis, perhaps one of the simplestdark and light echoes upon the Screen of the is illustrated in Theassumption in 3 picture tube is substantially that which would iS thattransmission is carried out in accordance obtain if no echoes were beingreproduced at alL with the single-carrier system, where Video In aconventional television system based on 30 and synchronizing Signals aretransmitted a complete frames per second, there will be fifteen commoncarrier frequency. Assume that a dark complete echo alternations secondeach echo, Such as is shown in name 1 of alternation consisting of oneframe having a dark is obtained- ,Thls echo extends from top echofollowed by a frame with a light echo. Aptom of the picture as shown,for each individual parenuy the rapid Substitution of light echoes lineof am includes echo of the type for dark echoes, and vice versa, causesthe eye to m detail] hne of and 7- echo average the echo effects and tosubstantially m each of the lmes W111 of course m ignore the individualecho images themselves. stantially identical positions relative to theedge Where an alternate carrier television system is of the picture. Iftransmission were carried out employed, 6" one in which the video and inthe conYentional manner frame and chronizing signals are transmitted atdifferent an Succeedmg frames f present the same carrier frequencies.the echoed synchronizing appearance as echo COnCemed' as frame signalbeats with the directly received video sig- Accotdlpg the P inventionnal to produce an alternating echo, as has aleveri transmlsslfm earnedout m ready been explained with reference to line D of ner that aperiodic reversal of phase relation Fig 2' when this occurs the screenof the takes place between the echo carrier and the tube (which maybemade up of 525 hues) video carrier, causing each frame having a darkmay present the appearance of either one f the echo m be followed by aframe having a light frames illustrated in Fi 4 (depending upon the esuch as that shown in frame 2 of a initial phase relation between theecho and the s y be p ed at the transmitting direct signal). Here theechoes are seen to consta ion y han in carrier phases 0 polarities sist,of a series of narrow vertical alternating at predetermined times. Forexample. a dark and light bars. An echo of the nature of numbered framesmight be transmitted in the that shown in frame No. 1 of Fig. 4 may besubconventional manner with no carrier phase stantially cancelled byalternating with it an echo of the nature of that shown in frame 2, thelatter echo being the reverse or the conjugate of the other.

The same general form of signal which was used to produce thealternating echo efiect described with reference to Fig. 3, i. e., thesignal represented in Fig. 7 (a) can also be employed to give thealternating sequence illustrated in Fig. 4. However, since the systemdescribed with reference to Fig. 4 is an alternate carrier system, itwill be understood that for the duration of each of the synchronizingpulses shown in Fig. 7 (a), the carrier frequency will be shifted inaccordance with the practice in alternate carrier systems. Means foraccomplishing these operations will be described in greater detailhereinafter.

We have found that cancellation of echoes can be made even moreeffective if, in each frame, the echo is broken up into a series ofalternate light and dark areas as illustrated in Figs. and 6. Thesefigures show patterns that may advantageously be employed inconventional and in alternate carrier systems respectively. Patterns ofthis character may be obtained by using signals in which carrier phasereversal is effected at the transmitter at approximately the beginningand the end of each of the cross-hatched intervals shown in the signalrepresentations (b) to (e) of Fig. 7. With signals of this type, thecharacter of the echo (1. e. whether dark or light) changes forsuccessive lines in time sequence. In a conventional interlaced scanningsystem, the eifect produced will be similar to that illustrated in Fig.5 in which the lines are numbered from I to l8 in time sequence for twocomplete frames, 1. e.. four complete fields. In line I, it is assumedthat the phase relation between the synchronizin signal echo and thevideo carrier is such that a dark echo results. In line 2 (which in aninterlaced system is spaced from line i by the width of one line), thisphase relation is reversed to produce a light echo, reversed again inline 3 to produce a dark echo, and so on for five lines to produce thefirst field. The second field, comprising lines 6 to 9, is transmittedwithout change in sequence so that the scanning pattern, or raster as itis called, consists of a plurality of pairs of lines with alternatelydark and light echoes, as illustrated in Fig. 5, frame No. 1. In frameNo. 2, this process is continued without change in sequence, the firstfield of frame 2 comprising lines II] to M, while the second fieldcomprises lines l5 to IE. It will be seen, however, that because eachframe includes an odd number of lines, the dark echoes in frame No. 2occupy those parts of the raster which in frame No. 1 were occupied bythe light echoes, and conversely. Since these frames are effectivelysuperimposed at short intervals in transmission, the echoes tend tocancel as far as the observer is concerned. The system illustrated inFig. 5 has the advantage over that in Fig.3 that it breaks up the echosignal so completely as to substantially eliminate all trace offickering in even those locations where echo signals are very strong.Effectively, the system of Fig. 5 interlaces the echoes in both time andspace relation.

Where the last-described system of echo interlacing is employed in analternate carriersystem, the echoes are further broken up in the mannerillustrated in Fig. 6, which may be regarded as the result of acombination of the methods employed in obtaining the rasters shown inFigs. 4 and 5. Of the several systems illustrated in Figs. 3 to 6, thesystem of Fig. 6 may be regarded as the preferred one. When the simplersystem of Fig. 3 is employed, and where very strong echo signals areencountered, it may be found that a trace of flicker may be visible as aresult of the 15 cycle alternation between relatively large unbrokenareas of light and dark echoes. (The flicker has a frequency of 15cycles per second, because there are 30 frames per second, 15 of whichhave a dark echo and 15 of which have a light echo.) If this flicker isdeemed objectionable, it may be eliminated or greatly reduced byemploying the system of Fig. 6 in which the general or overallillumination of the echo area remains substantially constant from frameto frame, as well as from field to field,

It has already been explained, in general, how echo cancellation may beobtained by successively reversing the phase of the echo with respect tothe picture carrier in time relation. Specific examples showing justwhen these phase reversals may be made, to secure echo cancellation ofthe types described with reference to Figs. 5 and 6, are illustrated inFig. '7 (b) to (e) inclusive. In these illustrations, the crosshatchedportions of the signals may be regarded as representing one arbitraryphase relation, the open portions of the signals representing asubstantially opposite phase relation.

In Fig. '7 (b), at time t1, the carrier phase is reversed and ismaintained in that relative phase until time its, at which time thecarrier phase is again reversed to bring it back to its originalrelative phase. The following video interval, blanking and synchronizinginterval, and second video interval are transmitted without change inrelative carrier phase, but at time is the carrier phase is againreversed, and is returned to its original relative phase at the end ofthe blanking signal at time 154. This cycle of events may be continuedwithout interruption from frame .to frame, although it is preferred tosuspend these phase changes during a portion of each vertical blankingand synchronizing interval so as to avoid any possibility of impairmentof interlacing. Preferably, the phase changes are suspended for the ninelines following the beginning of the vertical blanking period.

It is not necessary that the desired phase changes be effected preciselyat the beginning and end of the selected blanking pulses as shown inFig. 7 If desired, these changes may be made to occur at the beginningand end of the synchronizing pulses, or at some time within the blankingsignal intervals preceding and succeeding the synchronizing signalsthemselves. The latter system of timing is illustrated in Fig. '7 (e).Where phase changes are effected to coincide in time with thesynchronizing signals, rather than with blanking signals, it followsthat echo cancellation will be secured only for the synchronizing signalechoes, but since echoes of the synchronizlng signals are by far themost important, particularly in an alternate carrier system, the choicebetween the various timing sequences may be found to be largely one ofconvenience.

Referring generally to the signals represented in Fig. '7 at (b) and(e), it will be seen that every otherblanking and synchronizing signalis transmitted with its carrier phase reversed relative to the phase ofthe carrier during the rest of the television signal. Consequently, ifthe synchronizing pulse transmitted in the interval t1t2 is received asan echo during the immediately following video line interval, an echoimage will appear on the television screen for that particular line. Ifthis echo be a light one, indicated by the circled minus sign, the echoin the following line (in time sequence) will be a dark one, as isindicated by the circled plus sign. That the "sign of the echo will bediil'erent in the two cases will be seen from the fact that in one casean echo of one phase will beat with a video line of opposite phase,whereas in the other case, the echo and video line are of like phase.Thus, the echo image alternates from dark to light to dark, and so on,from line to line in time sequence, this being indicated in Fig. '7 (b)by the alternating plus and minus signs.

In the foregoing, cases have been described where the echo arrivesalternately in-phase and out-of-phase with the video signal. Obviously,of course, there will be instances wherein the echo will arrivealternately, leading and lagging the video signal, for example, by 90degrees. Where this occurs, the echo is of little importance, since theresultant of a strong signal (the direct signal) and a weak signal (theecho) differing 90 in phase is not substantially different in magnitudefrom the strong signal.

Another phase changing sequence capable of producing an echo image whichalternates from line to line in time sequence, is illustrated in Fig. 7Here the phase of the carrier is reversed after alternate synchronizingor blanking pulses, i. e., after every second pulse. Thus, at t1, theend of the first blanking interval, the carrier phase is reversed, butno further reversal in relative phase occurs until time t2, whichcorresponds to the end of the third blanking interval, and again at t3,the end of the fifth blanking interval, and so on. Here again the echoimage will alternate from light to dark, etc., as indicated by thecircled plus and minus signs.

In Fig. 7 (d) the carrier phase is reversed for alternate video (line)periods, the phase reversals taking place at times t1, ta, ta, etc., asindicated. -It will be seen that this procedure will also produce analternating echo similar to those produced by the signals of Fig. 7 (b)to (e) inclusive. In all of these variations, it should be understoodthat the phase changes are not necessarily made precisely at thebeginning and/or end of the blanking periods, but may be effected withinthe blanking periods, as illustrated in Fig. 7 (e), or may coincide withthe beginning and/or end of the synchronizing pulses themselves.Similarly, it should be understood that the invention is not limited tothe specific methods of echo cancellation illustrated in Fig. 7, sinceother suitable sequences of phase reversal may be utilized by thoseskilled in the art without departing from the methods and teachings ofthis invention.

From the illustrations of Fig. 7, it will be seen that the carrier phaseis preferably reversed at least times per second, where L is the numberof picture lines per frame and F is the number of complete framestransmitted per second.

Our invention may be put into effect by means of the transmitting systemshown diagrammatically in Fig. 8. For purposes of explanation, analternate carrier type of television system has been selected, but itwill be understood that the present invention is likewise adapted foruse with the more conventional fixed-frequency systems. An oscillator IQof controllable frequency serves as the primary source of carriersignal. If de- 050i UH HUUl sired, this source may operate at asubmultiple of the desired carrier frequency, the desired carrierfrequency being obtained by passing the wave from the source l9 througha, suitable frequency multiplier circuit 20. The carrier derived fromthe unit 20 may then be supplied to a suitable phase-reversing means 2|whose reversing operation is controlled in response to a keying signalfrom a source 22. The operation and construction of the units 2| and 22will be described in detail hereinafter. The carrier signal output ofthe unit 2! may then be passed through a suitable modulated amplifierstage 23, thence through the sesqui-side-band filter 24, and finally toa suitable antenna or radiating system 25.

Amplitude modulation of the carrier wave may be accomplished by means ofa conventional modulator stage 26 supplied with video, synchronizing,and blanking signals from the source 21. During the synchronizing signalintervals the fre-- quency of the carrier wave may be shifted to providealternate carrier transmission of the kind described in theabove-mentioned copending application of Frank J. Bingley. This may beaccomplished by applying synchronizing signals from the source 21, byway of the path 28, to the frequency shifting device 29, which isconnected to the oscillator stage IS in such a manner as to control thefrequency thereof. The frequency shifting device 29 may be of thereactance tube variety, and the circuits employed may be of the typedisclosed in the copending application of David B. Smith, Serial No.401,494, filed July 8, 1941. In the operation of a typical alternatecarrier system (disregarding for the moment the functions of the units2| and 22), the normal carrier frequency, which is held constant duringthe video and blanking portions of the signal, may be established at67.25 megacycles (to take a typical example). During the synchronizingsignal intervals, this carrier frequency may be shifted to anotherpredetermined frequency, for example, to 68.25 megacycles, and'since thetelevision re ceiver is designed to respond both to the frequency and tothe amplitude of the carrier during the synchronizing interval, asynchronizing pulse is produced in the television receiver which is ofsubstantially greater amplitude than that produced in a fixed frequencysystem under similar circumstances.

The phase reversals contemplated in the present invention may beproduced in the transmitting system of Fig. 8 by means of thephase-reversing device 2| which is capable of producing a pair of wavesof carrier frequency, but whose relative phases difler by substantiallydegrees. The device 2| is further characterized by the provision ofmeans for selecting either of these waves to the exclusion of the other,in response to a control or keying signal. This keying signal may bederived from the unit 22 which comprises the circuit means necessary forgenerating the proper keying signal in response to signals derived fromthe source 21. Where it is desired to employ the phase reversal sequenceillustrated in Fig. 7 (b), only the blanking signals need be supplied tothe keying signal source 22 from the source 21. The unit 22 may then beconstructed and arranged to supply a keying signal comprising impulseswhose duration and timing correspond to the duration and timing of everysecond blanking signal, i. e., to the cross-hatched signals of Fig. 7(b).

The operation of the system of Fig, 8 may be most readily described withreference to the signal representation of Fig. 7 (b). Commencing at timet: transmission is carried on at the normal carrier frequency (e. g.67.25 mc.) until the beginning of the synchronizing interval at time ta.At time ta, in accordance with the principles of alternate carriertransmission, the frequency of the carrier is shifted a substantialamount (e. g. to 68.25 mc.), returning to the normal carrier frequencyat time ft, the end of the synchronizing interval. At time ta, thebeginning of the third blanking interval in the illustration, thephasereversing device 2| of Fig. 8, in response to a keying signal fromunit 22, causes a sudden reversal in the phase of the carrier. At timeto the carrier frequency is again shifted (to 68.25 mc.), returning tonormal (67.25) at time ta. At time t4 the carrier phase is reversedthrough the agency of the device 2| of Fig. 8. The cycle of eventsoccurring during the period t2t4 is repeated during the period t4-ts,and so on, being interrupted only during the vertical blanking period,which period is not illustrated in the signal representations of Fig. 7.It will be understood that while this periodic interruption of thephasereversing operation is preferred, it is not necessary to theoperation of the present invention.

Reference is now made to Fig. 9 in which there is shown a schematicdiagram of a circuit adapted for use in the controllable phase-reversingmeans 2| and the keying signal source 22 of Fig. 8,

The controllable phase-reversing means 2| illustrated in Fig. 9comprises a pair of vacuum tube amplifiers V1 and V2 having their inputgrids connected in push-pull relation to the balanced secondary windingof the carrier input transformer 30, and their anodes connected inparallel to the interstage transformer 3|. In the operation of thiscircuit the tubes V1 and V2 are differentially biased in such a manner(to be explained in detail hereinafter) that only a selected one of thetubes is operative at any given time. If the tube V1 is operative, thesignal supplied to the transformer l8 will be in relatively reversedphase to the signal supplied in the event that tube V2 is operative andV1 inoperative. This follows from the fact that the grids of V1 and V2are connected to opposite ends of the split secondary of transformer 30,whereas the anodes of V1 and V2 are connected together and to the upperor high potential end of the primary winding of transformer 3|. Sincethe circuit included in the unit 2| must transfer not only the normalcarrier frequency but also the shifted or synchroni'zing carrierfrequency, it is preferred that each of the transformers 30 and 3|, aswell as the output transformer 32, be capable of transferring eithercarrier frequency with equal facility. Accordingly, these transformersmay be suitably damped and overcoupled to provide the desired band-passcharacteristic.

The keying signal unit 22 is a device for controlling the bias of tubesV1 and V: of unit 2| in such a manner that a selected one of the saidtubes is rendered operative while the other is rendered inoperative, orvice versa. The bias of the tubes V1 and V2 are controlled by the tubesV and V6 respectively, the cathode of V1 being connected by way of thelead 33 to the cathode load 34 of the tube V5. Similarly, the cathode ofV2 is connected by way of the lead 35 to the cathode load 36 of the tubeVs. Neither of the tubes V5 and V6 is here provided with a plate circuitload, their screens and anodes being connected directly to the positivehigh potential supply terminal B+.

The control grids of tubes V5 and V6 are condenser-coupled to the anodeand cathode loads 31 and 38, respectively, of the signal invertingdriver tube V4. Preferably, the anode and cathode loads 31 and 38 aresubstantially equal in magnitude so that the signals applied to thecontrol grids of V5 and V6 will be not only opposite in phase, but equalin magnitude as well. The operation of tube V4 may be controlled by asuitable source of control pulses represented by the rectangle 39. Tocarry on the preceding description of a system for supplying a signal ofthe type described with reference to Fig. 7 (b), the device 39 maycomprise a circuit adapted to be energized by the blanking signal andcapable of supplying to the control grid of V4 a signal similar to theblanking signal, but having only half the number of pulses per second.In Fig. 9 the rectangle 39 has, accordingly, been referred to as anAlternate pulse rejector. Circuits capable of performing such a functionare known to those skilled in the art. and, consequently, it is deemedunnecessary to provide herein a detailed description of this device. Byway of example, however, it may be said that satisfactory devices andmethods for selecting or rejecting predetermined pulses are disclosed inthe F. J. Bingley Patent No. 2,171,536 (e. g., see Fig. 5), and thecopending application of F. J. Bingley, Serial No. 357,179 (e. g., seeFig. 4).

Assume that the tube V4, is normally biased substantially toplate-current cut-off (i. e. during the longer intervals tz-ta, t4--t5,etc. of Fig. 7 (b), but is driven substantially to plate-currentsaturation during the shorter intervals ti--tz, tat4, etc., by thepulses received from the device 39. Under these conditions there will beestablished across the cathode load 36 a signal having the wave shapeand polarity (phase) of the signal applied to the grid of V4, whileacross the cathode load 34 there will appear a signal having like waveshape, but opposite polarity. These signals (i. e. the signals appearingat the cathodes of V5 and V6) may then be employed as keying signals todifferentially render operative and inoperative tubes V1 and V2 in thephase reversing unit 2|, for the purpose hereinbefore described.

It has already been stated that the carrier phase reversals arepreferably suspended during a portion of the vertical synchronizingintervals. This is readily accomplished through the agency of one of thesignals supplied by conventional generators of standard RMAsynchronizing signals. This signal is a substantially rectangular signaloccurring at the rate of sixty per second, and having a duration ofapproximately nine line periods. The signal in question starts insynchronism with the vertical blanking pulse, but has a duration of onlynine line periods, and, hence, extends about three lines beyond the endof the vertical synchronizing pulse. Its interval corresponds to theinterval occupied by the equalizing pulse train of the standard RMAsignal. Accordingly, this 9-line signal (which can be derived from unit21 of Fig. 8) may be supplied to the keying signal source 22 along withthe modified blanking signal supplied by the alternate pulse rejector 39of Fig. 9. The signals may be added together by means of a conventionalsignal combining circuit, of which many varieties are well known in theart.

The relation between the blanking, synchronizing, and keying signals,the operation of tubes V1 and V2, and the frequency and phase changesinvolved in the operation of the specific embodi- 178. TELEGRAPH, ocaleullUUl ment illustrated and described with reference to Figs. '7 (b), 8,and 9, may best be understood by referring to the explanatory drawingsof Fig. 10. The various functions here illustrated are all drawn againsta common time axis which employs the notations t1, t2, etc. already usedin Fig. '7 (b).

The first signal illustrated in Fig. is the synchronizing signal which,in Fig. 8, is transferred from the source 21 to the frequency shifter 29to produce the necessary frequency shift in the oscillator l9 to providethe desired alternatecarrier operation of the transmitter. This signalis also supplied (together with the video and blanking signals) to themodulator stage 26 to amplitude-modulate the carrier wave in the usualmanner. The second signal illustrated is the blanking signal which istransferred from the source 21 to the alternate pulse rejector 38 ofFig. 9. The output of unit 39 is the third signal illustrated in Fig.10. The keying signal derived from cathode load 36 of V6 is denoted K.S. 36 in Fig. 10. The keying signal K. S. 34, derived from cathode load34 of V5, is seen to be of similar wave form, but of opposite polarity.Signal K. S. 34, which is applied to the cathode of V1, renders tube V1inoperative during the periods tzt3, t4-t5, etc. but operative duringthe periods t1-tz, t3t4, etc., as is indicated by the cross-hatchedareas in line V1 of Fig. 10. On the other hand, signal K. S. 36 which isapplied to the cathode of V2, renders the tube V2 inoperative during theperiods t1-t2, ifs-t4, etc., but operative during the periods tz-ts,t4-t5, etc., as represented by the cross-hatched areas in line V2 ofFig. 10.

The phase and frequency changes undergone by the carrier wave areillustrated in the diagrams designated Relative phase and Carrierfrequency, respectively. During the video (line) intervals, as well asduring those portions of the blanking intervals which immediatelyprecede and succeed the synchronizing signals, the carrier frequency isrepresented as being fixed at frequency iv (e. g. 67.25 mc.). During thesynchronizing signal intervals, the carrier frequency is represented asbeing shifted to a substantailly different frequency is (e. g. 68.25mc.). This periodic carrier frequency shift is in accordance with thealternate carrier system of transmission already referred to.

During the longer intervals t2t3, tr-ts, etc., the relative phase of thecarrier may be regarded as substantiallly fixed at some arbitrary valueThe phase is periodically reversed, however, and, accordingly, duringthe shorter intervals tr-tz, t;t4, etc., the relative phase of thecarrier is advanced to a value 180. Of course, it might just as well beretarded to a value 180. These phase shifts are effected duringalternate blanking signal impulses in accordance with the echocancellation method described with reference to Fig.7 (b).

Strictly speaking, the phase" of an altemating wave varies continuouslyat the rate of 360 degrees per cycle. In the present invention, however,we are not concerned with this kind of phase variation, but rather withthe phase difference between a directly received carrier signal and anindirectly received carrier (echo) signal. It should be recognized,however, that even this phase difference" or "differential phase may bevariable. Thus in an alternate carrier system the phase relation betweenthe video carrier and the synchronizing signal carrier is constantlychanging, and it is this continuous phase change which produces the"echo beats and hence the alternating echoes described with reference toFigs. 2, 4, and 6. Consequently, in the foregoing description, and inthe claims, the term phase or relative phase" is employed as a measureof the kind of phase that concerns us most in the description of ourinvention. Thus, in the function designated Relative phase in Fig. 10,the regular phase progression from cycle to cycle of the R. F. carrieris ignored, as are also the less regular phase changes which may occuras a result of carrier frequency drift or variation, and only the phasereversals produced by the phase reversing means 2! of Figs. 8 and 9 arerepresented.

While the operation of the apparatus of Figs. 8 and 9 has been describedin detail only with respect to the echo cancellation method of Fig. '7(b), the foregoing example will enable one skilled in the art to putinto practice other echo cancellation methods, such, for example, asthose described with reference to Fig. 7 (a), (c), (d) and (e). Sincemethods for generating the necessary keying signals in response to theblanking and/or synchronizing signals are well known, it is not deemednecessary to refer to the details of this process. U. S. Patent No.2,171,536, issued to F. J. Bingley, discloses certain specific methodsand means for forming various types of recurrent pulses which may beadapted for use in the present invention.

The echo cancellation systems so far described may be referred to astwo-phase echo cancellation systems, inasmuch as they rely for theiroperation upon a system of phase reversals, i. e.. of successive phasechanges of 180 degrees. As has already been inferred in one of thestated objects of this invention, other phase changes are likewiseadapted for use in putting our invention into practice. Thus, it isreadily possible to employ threeand four-phase systems of phase changingif desired. In a two-phase system, the carrier phase may be caused tochange according to the order of 0, 180, 0, 180", etc. in somepredetermined time sequence, as described with reference to the drawingsof Fig. 7. In a three-phase system, the changes may take the order of0", 240, 0, etc. For example, in frame No. 1, the video andsynchronizing signals might be transmitted without change in carrierphase relations. In frame No. 2 the carrier phase during thesynchronizing intervals would be advanced (or retarded) 120 with respectto carrier phase during the video intervals. And during frame No. 3 thecarrier phase during the synchronizing intervals would be advanced (orretarded) 240 with respect to carrier phase during the video intervals.This corresponds, for the three-phase case, to the twophase caseillustrated in Fig. 3. In the threephase system of echo cancellation,three frames are required for each echo cancellation cycle, as comparedto two frames for the two-phase system. In the three-phase system, itis, therefore, particularly advantageous to interlace the echo in bothtime and space relation, as in Figs. 5 and 6, to eliminate theoccurrence of an objectionable 10 cycle echo flicker.

Reference is now made to Fig. 11 in which a three-phase circuit is shownwhich is capable of interlacing an echo in both time and space relation.As will be apparent the circuit of Fig. 11 represents the circuit ofFig. 9 adapted to threephase operation. It will be understood that inconverting the system of Fig. 8 from a twophase to a three-phase systemthe circuits of Fig. 11 would be substituted for the rectangles de noted2| and 22.

In Fig. 9, it will be recalled, a pair of carrier waves of oppositephase are derived from opposite ends of the center-tapped transformer30. In the circuit of Fig. 11 the equivalent function is performed bythe single-phase-three-phase converter 40. This device supplies in itsoutput a three-phase carrier whose phases are preferably balanced toground. Single-phase-threephase converters which are suitable for thepur poses of this invention are fully described and illustrated in Hund,Phenomena in high-frequency systems, first edition, 1936, pages 144-146.Carrier signals of phases #1, #2 and #3 are applied respectively to theinput circuits of carrier transfer tubes T1, T2 and T3, whose anodes areconnected to a common output circuit 3i. As in Fig. 9, the operation ofeach of the tubes T1, T2, T3, is controlled by means of correspondingcontrol tubes T4, T5 and Te. Preferably this control is of such a naturethat at any one time only one of the tubes T1, T2 and Ta is operative totransfer a carrier of selected phase to the common output circuit 3|.The sequence of operation of the control tubes T4, T5 and To isdetermined by control signals El, E2 and E3 which are applied to theinput circuits of the said control tubes. Thus if a particular controlsignal, let us say E1, is at the level designated the correspondingcarrier transfer tube Tl will be operative, and will transfer phase #1carrier to the output circuit 3|. On the other hand when the controlsignal E1 is at the level designated the carrier transfer tube T1 isrendered inoperative, and the carrier is transferred through tube T2 orT3, depending upon which of the latter tubes are operative. The samemode of operation holds with respect to signals E2 and E3, and carriertransfer tubes T2 and Ta.

In Fig. 11 the signal f represents, for the three-phase condition ofoperation, what the signal b of Fig. 7 represents for two-phaseoperation. With respect to the signal f and to the time scale showntherewith, it will be seen that phase #1 is maintained for video andblanking alike until time 112, at which time the phase is advanced (orretarded) by 120 electrical degrees. phase is again returned to theoriginal phase, phase #1. At time ta the phase is again changed, thistime by 240 degrees and this condition is maintained until the end ofthe blanking interval, at which time the phase is again returned to theoriginal condition, 1. e. to phase #1. This cycle of events is repeatedas is indicated in the diagram which appears above the signal f.

The above-described phase changes may be accomplished by the applicationof the control signals E1, E2 and E3 to the control tubes T4, T5 and T6respectively. While the signal E1 is at the operative level 0 (and thesignals E2 and E3 at the inoperative levels in), phase #1 carrier istransferred to the output circuit 3| by way of the carrier transfer tubeT1. At time is, and for a time equal to the duration of the blankingsignal, the tube T1 is rendered inoperative while the tube T2 isrendered operative through the agency of the signal E2 and the controltube T5. Immediately thereafter tube T1 again becomes operative, but attime t: is rendered inoperative, while the tube Ta is caused to becomeoperative through the agency of the signal E3 and the control tube Te.At the end of the third At the end of the blanking interval theillustrated blanking signal the tube T3 is rendered inoperative, whilethe tube T1 is again rendered operative. In a thirty-frame-per-secondtelevision system this cycle of events may be repeated each one-tenth ofa second.

The control signals E1, E2 and E3 are readily generated by means ofdevices which are well known in the art. The signal E2, for example, maybe provided by means of a device which, when provided with blankingsignals from the transmitters blanking signal source, will select everythird one of the said blanking signals. Thus signal E2 may be regardedas representing blanking signals Nos. 2, 5, 8, and so on, while controlsignal E3 may be regarded as comprising blanking signal Nos. 3, 6, 9,and so forth. Circults and means for producing such control signals arefully described in the aforementioned F. J. Bingley copendingapplication and issued patent. Control signal E1 represents the sum ofcontrol signals E2 and E3 (added together in a suitable combiningcircuit), inverted to produce the proper sequence of operation ofcontrol tube T4.

In conventional television transmission systems it is customary toemploy a raster having an odd number of lines, for example 525 lines. Inthe three-phase system just described it is desirable that, in order toproceed with an uninterrupted phase-changing sequence, the total numberof lines per frame be not divisible by the numeral 3. Thus a 605-lineraster would be acceptable in a three-phase echo-cancellation system,since 605 is an odd number not divisible by 3. The present inventionhowever is not to be limited, for the three-phase case, to a rasterhaving an odd number of lines not divisible by 3, inasmuch as it isalways possible to control the phase of the echo image by advancing (orretarding), by one line and blanking period, the

sequence of operation of the control tubes T4, T5 and T6 at thebeginning of each frame. This would be done so that a given black echowould occupy the same position, on a given line, only once during aperiod of three frames.

Thus far our invention has been described with particular reference totelevision transmission systems in which the video signal is transmittedby amplitude modulating the carrier. The invention is also adapted foruse, however, in systems wherein the video signal is transmitted byfrequency modulating the carrier. In fact, because of the particularlyobnoxious appearance of the echo images in PM television transmissionsystems, some means for eliminating or cancelling echoes is especiallyto be desired.

In FM systems the widths of the echo bars are not fixed as they are inan alternate carrier AM system (e. g. see Figs. 4 and 6). Instead theyvary in width from time to time in accordance with the changingillumination of the picture in the screen area afiected by tlte echo.This is because in an FM television system the instantaneous carrierfrequency is a fnnction of illumination, and, consequently, the numberof beats (which produce the echo bars) between the delayed synchronizingsignal (1. e. the echo) and the received picture signal varies withpicture illumination. Thus, a the illumination of the picture varies ina given area-e. g. as caused by the movement of actors or vehicles, orby movement of the television cameraF-the echo will present anever-varying and moving image, which because of its motion is much moreobjectionable to the observer than the motionless or fixed IlUlLb-hull"! II I.

UCCH Ll HUUI echoes encountered in AM systems of television. The resentinvention can greatly reduce the effect of these echoes in spite oftheir movement, because in general this movement (of the echo over thescreen) takes place slowly enough so that between identical lines insuccessive frames there is sufficient similarity as to echo imageposition to enable an alternating dark-and-light echo in one frame to bereplaced by a substantially correspondingly-placed alternatinglightand-dark echo in the immediately following frame. Thus, it will beseen that our invention is adapted not only to systems wherein the echoproduces a substantially fixed pattern upon the picture tube screen, butalso to systems in which the echo pattern, while varying substantiallyfrom minute to minute, or second to second, changes only slightly, or toa negligible degree, from frame to frame.

Although our invention has been described and illustrated with referenceto certain preferred embodiments, it should be understood that widealterations and modifications may be made within the scope of thisinvention as defined in the appended claims.

We claim:

1. In a carrier wave television transmission system, the method ofreducing the deleterious effects of echo signals on the desired signal,which comprises periodically altering the relative carrier phase ofselected carrier intervals to produce echo images of contrastingcharacteristics in successive frames.

2. In a carrier wave television transmission system, the method ofsubstantially diminishing the deleterious effects of synchronizingsignal echoes on the desired picture signals, which comprisesperiodically changing the phase of the synchronizing signal carrierrelative to the phase of the video signal carrier to produce echo imagesof periodically changing characteristics.

3. In a carrier wave television transmission system wherein videosignals are transmitted at one carrier frequency and synchronizingsignals are transmitted either at the same carrier frequency or at adifierent carrier frequency, the method of reducing the deleteriouseffects of beats between the desired video signals and echoes Of thesynchronizing signals, which comprises periodically changing the phaseof the synchronizing signal carrier relative to the phase of the videosignal carrier.

4. In an alternate carrier television transmission system wherein videosignals are transmitted at one carrier frequency and synchronizingsignals are transmitted at a different carrier frequency, the method ofreducing the deleterious effects of carrier frequency beats between thedesired video signals and echoes of the synchronizing signals, whichcomprises reversing the phase of said beats in successive frames of thetransinitted picture.

5. In a carrier wave television transmission system, the method ofreducing the deleterious efiects of echo signals on the desired signal,which comprises reversing the carrier phase at least times per second inpredetermined sequence prior to transmission, thereby to cause echoimages of complementary characteristics to appear upon a receiver'spicture viewing screen in alternating per frame, and F is the number ofcomplete frames transmitted per second.

6. In a television system, the method of generating a television signalwhich will ensure substantially echo-free reception, which comprisesgenerating a carrier wave, periodically altering the relative phase ofsaid wave in a predetermined time sequence, and modulating said wave inaccordance with the intelligence to be transmitted.

7. In a television system, the method of generating a television signalwhich will ensure substantially echo-free reception, which comprisesgenerating a first carrier wave, generating a second carrier wave oflike frequency but of substantially opposite phase, alternatelyselecting one and then the other of said waves in a predeterminedrepetitive time sequence, and modulating the selected wave in accordancewith the intelligence to be transmitted.

8. In a television system, the method of generating a television signalwhich will ensure substantially echo-free reception, which comprisesgenerating a first carrier wave, generating a second carrier wave oflike frequency but of 0pposite phase, selecting the first of said wavesduring even frame periods, selecting both of said waves in alternatingsequence during odd frame intervals, and modulating the selected wave inaccordance with the intelligence to be transmitted.

9. In a television system, the method of generating a television signalwhich will ensure substantially echo-free reception, which comprisesgenerating a first carrier wave, generating a second carrier wave oflike frequency but of opposite phase, selecting the first of said wavesduring video intervals and during even synchronizing intervals,selecting the second of said waves during odd synchronizing intervals,and modulating the selected wave in accordance with the intelligence tobe transmitted.

10. In a carrier wave television transmission system, the method ofreducing the deleterious effects of synchronizing signal echoes on thevideo signal, which comprises reversing the phase of the transmittedcarrier wave before and after every second line-synchronizing pulse inthe picture interval.

11. In a carrier wave television transmission system, the method ofreducing the deleterious effects of synchronizing signal echoes on thevideo signal, which comprises reversing the phase of the transmittedcarrier wave after every second line-synchronizing pulse in the picturein terval.

12. In a carrier wave television transmission system, the method ofreducing the deleterious effects of synchronizing signal echoes on thevideo signal, which comprises reversing the phase of the transmittedcarrier wave before and after every second video line period in thepicture interval.

13. In a carrier wave television transmission system, the method ofsubstantially diminishing the deleterious effects of synchronizingsignal echoes on picture signals, which comprises changing, in apredetermined time sequence, the differential carrier phase betweenhorizontal synchronizing signals and the succeeding line signals.

14. In a carrier wave television transmission system, the method ofsubstantially diminishing the deleterious effects of synchronizingsignal sequence, where L is the number of picture lines 15 echoes onpicture signals, which comprises reversing, in a predetermined timesequence, the differential carrier phase between horizontalsynchronizing signals and the succeeding line signals.

15. In a carrier wave television transmission system, the method ofsubstantially diminishing the deleterious effects of synchronizingsignal echoes on picture signals, which comprises changing, in apredetermined time sequence, the differential carrier phase betweenhorizontal synchronizing signals and the succeeding line signals, saiddifferential carrier phase changes being of such magnitude that a smallintegral number of them produce a phase rotation of substantially 360degrees.

16. In a carrier wave television transmission system, the method ofsubstantially diminishing the deleterious effects of synchronizingsignal echoes on picture signals, which comprises periodically changingthe differential carrier phase between the line synchronizing signalsand the succeeding line signals, and interrupting said periodicdifferential carrier phase changes during vertical synchronizing signalperiods.

17. In a carrier wave television transmission system wherein thetransmitted signal includes video line periods, horizontal synchronizingsignal periods, and horizontal blanking signal periods immediatelypreceding and succeeding said synchronizing signal periods, the methodof substantially diminishing the deleterious effects of synchronizingsignal echoes on picture signals, which comprises changing, in apredetermined time sequence, the differential carrier phase betweenselected horizontal synchronizing signals and the succeeding linesignals, said changes being effected during selected horizontal blankingsignal periods.

18. In a carrier wave television transmission system in whichsynchronizing and picture signals are transmitted at dissimilar carrierfrequencies, the method of effectively eliminating the echo imagesproduced by the beating of the picture signal with echoes of thepreceding synchronizing signal, which comprises periodically reversingthe differential carrier phase between preselected synchronizing signalsand picture signals, the periodicity selected being such that pairs ofcomplementary echo images are produced upon the viewing screen at thereceiver within time intervals substantially equal to that of thepersistence of vision.

19. In a 30-frame-per-second television transmission system, the methodclaimed in claim 18, wherein pairs of complementary echo images areproduced each fifteenth of a second.

20. In a. television transmitting system including a source of carrierwave oscillations, apparatus for reducing the deleterious effects ofecho signals on the desired signal, comprising controllable means forperiodically shifting the relative phase of said oscillations inresponse to a control signal, the magnitude of each phase shift beingsubstantially 360/11. electrical de rees, where n is a small integerother than 1, and a source of control signals connected to said phaseshifting means to control the periodicity of said phase shifts.

21. A television transmitting system as claimed in claim 20, wherein theinteger n is the number 2.

22. In a television transmitting system including a source of carrierwave oscillations, apparatus for reducing the deleterious effects ofecho 'signals on the desired signal, comprising controllable means forreversing the relative phase of said oscillations in response to acontrol signal, means for modulating the oscillations derived from saidphase reversing means in accordance with an intelligence signal, asource of synchronizing and blanking signals, a control signal sourceconnected to and deriving signals from said second-named source, and aconnection between said control signal source and said phase reversingmeans for eifecting carrier phase reversals in accordance with apredetermined function of signals derived from said secondnamed source.

23. In a television transmitting system. apparatus for reducing thedeleterious effects of echo signals on the desired signal, comprisingmeans for producing carrier wave oscillations of predetermined frequencyand phase, means for producing other carrier wave oscillations of likefrequency but of substantially different phase. controllable meansconnected to both of said first two means for alternately selecting oneand then the other of said oscillations in response to a control signal,a source of timing signals, means for generating a control signalsynchronized with said timing signals, connections for applying saidcontrol signals to said controllable means to control the operationthereof, and a transfer channel for transferring the selectedoscillations to subsequent elements of the transmitting system.

24. A television transmitting system as claimed in claim 23, wherein thephase difference between the carrier wave oscillations is substantially360/n degrees, where n is a small integer other than 1.

25. In a television transmitting system including a source of carrierwave oscillations, apparatus for reducing the deleterious effects ofecho signals on the desired signal, comprising means connected to saidsource for deriving therefrom a first carrier wave of controlledfrequency and a second carrier wave of like frequency but substantiallyopposite phase, wave selector means responsive to a control signalforperiodically selecting and transferring one and then the other ofsaid carrier waves to a latter stage of said transmitting system, and asource of periodic control signals for timing the selective operation ofsaid wave selector means.

26. A television transmission system as claimed in claim 25,characterized in that the signals from said control signal source aresynchronized with the systems synchronizing signals.

27. A television transmission system as claimed in claim 25,characterized in that said control signals are timed to reverse, in apredetermined time sequence, the diflerential carrier phase betweenhorizontal synchronizing pulses and succeeding line signals.

28. In a television transmitting system, a source of carrier waveoscillations, means for modulating said oscillations with anintelligence signs, and auxiliary means operative upon said oscillationsfor producing upon the viewing screen at a television receiving stationan echo image of one characteristic during odd frame intervals and anecho image of a substantially complementary characteristic during evenframe intervals.

29. A television transmitting system as claimed in claim 28,characterized in the provision of additional means operative upon saidoscillations to produce echo images of alternating characteristicsduring each frame, whereby the echo is US! UN HUU effectively interlacedin both time and space relation.

30. In a television system, the method of transmission which comprisestransmitting video signals at one predetermined carrier frequency,transmitting synchronizing signals at a substantially different carrierfrequency, and reversing, in a, predetermined time sequence, thedifferential carrier phase between selected horizontal synchronizingsignals and the succeeding video signals, thereby substantially todiminish the deleterious effects of echoes of the synchronizing signalson the video signals.

31. In a television transmitting system, a source oi carrier waveoscillations, frequency control means connected to said source forcontrolling the frequency of said oscillations in response to a controlsignal, means for generating a control signal and for applying saidcontrol signal to said frequency control means to establish thefrequency of said oscillations at one predetermined value during videointervals and at a substantially difierent value during synchronizingintervals, phase control means connected to said source for controlling,in response to a timing signal, the phase of the oscillations derivedfrom said source, asource of periodic timing signals operativelyconnected to said phase control means, a source of video signals, andmeans operative during said video intervals for amplitude-modulatingsaid derived oscillations in accordance with the signals from saidlast-named source.

32. A television transmitting system as claimed in claim 31,characterized in that said periodic timing signals are synchronized withthe transmitters synchronizing signals, and are of such periodicity asto change the phase of said derived oscillations during predeterminedsynchronizing intervals with respect to the phase of said derivedoscillations during predetermined video intervals.

33. A television transmitting system as claimed in claim 31,characterized in that said periodic timing signals are synchronized withthe transmitters synchronizing signals, and are of such periodicity asto change the phase of said derived oscillations during predeterminedsynchronizing intervals with respect to the phase of said derivedoscillations during other predetermined synchronizing intervals.

34. In a television transmitting system including a, source of carrierwave oscillations, apparatus for reducing the deleterious effects ofecho signals on the desired signal, comprising phase control meansconnected to said source for controlling, in response to a timingsignal, the phase of the oscillations derived from said source, saidphase control means being constructed and arranged to produce relativelyinstantaneous phase shifts of substantially 360/11 electrical degrees,where n is a small integer other than 1, and a source of periodic timingsignals connected to said phase control means to control the operationthereof.

35. A television transmitting system as claimed in claim 34, wherein theinteger n is the number 3.

FRANK J. BINGLEY. WILLIAM E. BRADLEY.

